Mechanical properties of polyethylene resins depend on the molecular weight and molecular weight distribution, and the application field is selected accordingly. In general, the higher molecular weight of polyethylene resin provides the better mechanical properties thereof, but polyethylenes with high molecular weight have low fluidity to cause difficulties during the processing. This may result in serious disadvantage in extrusion or injection molding, which requires rapid deformation with high shear rate.
Thus studies have been performed on methods for improving mechanical properties of polyethylene product such as impact strength and creep property while maintaining the fluidity of polyethylene product, by copolymerizing ethylene with α-olefins such as 1-butene, 1-pentene, 1-hexene and 1-octene. Especially, the market requires the products having high impact resistance, which has been conventionally evaluated by falling dart impact strength or high rate impact strength depending on the process for preparation thereof (whether it is film formation or injection). When copolymerization of ethylene with α-olefin is carried out in order to improve the mechanical strength such as impact resistance as described above, however, rigidity is deteriorated because copolymer with relatively low density is obtained. Decreased rigidity of products intended for lighter and thinner ones may cause deformation when they are bent or stacked.
Accordingly, Japanese Patent Laid-Open No. 2000-72819 proposed ethylene homopolymers or copolymers of ethylene with not more than 10% by weight of C3˜C20 α-olefin having (1) intrinsic viscosity [η]=2˜6 dl/g; (2) density of 0.94˜0.970 g/cm3; (3) zerocia viscosity at 190° C. η0=2×107˜3×108 poise; (4) melt tensile strength (MT) and melt index under 21.6 kg of load (HLMI) satisfying the correlation: MT≧−12.4 log HLMI+20.5; and (5) high rate impact strength (HRI-IZOD) measured at −30° C. and HLMI satisfying the correlation: HRI-IZOD≧−log HLMI+1.15; which are manufactured by blown molding with restricted use for fuel tanks.
US Patent Publication No. 2006-0235147 proposes polyethylene having the density of 0.914˜0.945 with excellent optical properties, low content of extractable substances, and excellent falling dart impact strength and high rate impact strength, for package material such as food package film, bags and pouches.